Transcript Stationary Waves - St. Joseph's Anglo

Stationary Waves
http://www2.biglobe.ne.jp/~norimari/science/JavaEd/e-wave4.html
Stationary waves are produced by superposition
of two progressive waves of equal amplitude and
frequency, travelling with the same speed in
opposite directions.
Production of Stationary Waves
A stationary wave would be set up by
causing the string to oscillate rapidly at a
particular frequency.
If the signal frequency is increased further,
overtone patterns appear.
Properties of a stationary wave (1)
Stationary waves have nodes where there is no
displacement at any time.
In between the nodes are positions called antinodes,
where the displacement has maximum amplitude.
Vibrator
N
A
A vibrating loop
N
l
A
N
Properties of a stationary wave (2)
The waveform in a stationary wave does not move
through medium; energy is not carried away from the
source.
The amplitude of a stationary wave varies from zero
at a node to maximum at an antinode, and depends
on position along the wave.
b
c
d
i
a
e
h
f
a
g
b c d
e
t=0
i
t = ¼T
f g h
t = ¼T
t = ⅜T
t = ½T
Vibrations of particles in a
stationary wave
At t = 0, all particles are at rest because
the particles reach their maximum
displacements.
At t = ¼T,
Particles a, e, and i are at rest because
they are the nodes.
Particles b, c and d are moving
downward.
They vibrate in phase but with different
amplitude.
Particles f, g and h are moving upward.
They vibrate in phase but with different
amplitude.
Properties of a stationary wave (2)
All particles between two adjacent nodes
(within one vibrating loop) are in phase.
Video
1. Stationary waves (string)
2. Stationary waves (sound)
Modes of vibration of strings
http://id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html
Picture of Standing Wave
L
Name
1st Harmonic
L = ½λ1
or
f1 = v/2L Fundamental
2nd Harmonic
L = λ2
or
f2 = v/L
1st Overtone
L = 1½λ3 3rd Harmonic
or
f3 = 3v/2L 2nd Overtone
4th Harmonic
L = 2λ4
or
f4 = 2v/L
3rd Overtone
5th Harmonic
L = 2½λ5
or
f5 = 5v/2L 4th Overtone
Structure
1 Antinode
2 Nodes
2 Antinodes
3 Nodes
3 Antinodes
4 Nodes
4 Antinodes
5 Nodes
5 Antinodes
6 Nodes
Investigating stationary waves using
sound waves and microwaves
Moving the detector along the line between the wave
source and the reflector enables alternating points of
high and low signal intensity to be found. These are the
antinodes and nodes of the stationary waves.
The distance between successive nodes or antinodes can
be measured, and corresponds to half the wavelength λ.
If the frequency f of the source is known, the speed of
the two progressive waves which produce the stationary
wave can be obtained.
Reflector
Wave source
Detector
Resonant Frequencies of a Vibrating
String
From the experiment, we find that




There is a number of resonant frequencies
in a vibrating string,
The lowest resonant frequency is called the
fundamental frequency (1st harmonic),
The other frequencies are called overtones
(2nd harmonic, 3rd harmonic etc.),
Each of the overtones has a frequency
which is a whole-number multiple of the
frequency of the fundamental.
Factors that determine the fundamental
frequency of a vibrating string
The frequency of vibration depends on



the mass per unit length of the string,
the tension in the string and,
the length of the string.
The fundamental frequency is given by
1 T
fo 
2L 
where T = tension
 = mass per unit length
L = length of string
Vibrations in Air Column
http://www.walter-fendt.de/ph11e/stlwaves.htm
When a loudspeaker producing sound is placed
near the end of a hollow tube, the tube
resonates with sound at certain frequencies.
Stationary waves are set up inside the tube
because of the superposition of the incident
wave and the reflected wave travelling in
opposite directions.
Factors that determine the fundamental
frequency of a vibrating air column
The natural frequency of a wind
instrument is dependent upon


The type of the air column,
The length of the air column of the
instrument.
Open tube
Closed tube
Modes of vibration for an open tube
Picture of Standing Wave
L = ½λ1
f1 = v/2L
L = λ2
f2 = v/L
L = 1½λ3
f3 = 3v/2L
L = 2λ4
f4 =2v/L
L = 2½λ5
f5 = 5v/2L
Name
1st Harmonic
or
Fundamental
2nd Harmonic
or
1st Overtone
3rd Harmonic
or
2nd Overtone
4th Harmonic
or
3rd Overtone
5th Harmonic
or
4th Overtone
Structure
2 Antinodes
1 Node
3 Antinodes
2 Nodes
4 Antinodes
3 Nodes
5 Antinodes
4 Nodes
6 Antinodes
5 Nodes
Modes of vibration for a closed tube
Name
Picture of Standing Wave
1st Harmonic
or
Fundamental
3rd Harmonic
L = ¾λ3
or
f3 =3v/4L 1st Overtone
5th Harmonic
L = 1¼λ5
or
f5 =5v/4L 2nd Overtone
L = 1¾λ7 7th Harmonic
or
f7 = 7v/4L 3rd Overtone
L = 2¼λ9 9th Harmonic
or
f9 =9v/4L 4th Overtone
L = ¼λ1
f1 = v/4L
Structure
1 Antinode
1 Node
2 Antinodes
2 Nodes
3 Antinodes
3 Nodes
4 Antinodes
4 Nodes
5 Antinodes
5 Nodes
The quality of sound (Timbre)
http://surendranath.tripod.com/Harmonics/Harmonics.html
The quality of sound is determined by the
following factors:
The particular harmonics present in addition to the
fundamental vibration,
 The relative amplitude of each harmonic,
 The transient sounds produced when the vibration is
started.
resultant
Fundamental
1st overtone

2nd overtone
3rd overtone
Chladni’s Plate
Chladni’s plate is an example of resonance in
a plate.
There are a number of frequencies at which
the plate resonate. Each gives a different
pattern.